For Immediate Release
Known as "catch bonds," the adhesion mechanism displays surprising
behavior, prolonging rather than shortening the lifetimes of bonds between
specific molecules as increasing force is applied. Proposed theoretically
nearly 15 years ago, catch bonds could help explain how the body regulates
the activity of white blood cells, which must flow freely through blood
vessels -- yet bond to injury sites despite blood flow forces.
Understanding how catch bonds work could offer drug designers a new target
for anti-inflammatory and anti-thrombosis compounds, and potentially provide
a new approach to controlling the metastasis process that cancers use
"Before the experimental demonstration of catch bonds, we tended
to think that force could regulate biochemical reactions only in one direction,"
Zhu, a professor in the Woodruff
School of Mechanical Engineering at the Georgia Institute of Technology.
"This work demonstrates that force can alter the rate in the other
direction, depending on the type of interaction. In this post-genome era,
we need to know more about how proteins interact with one another and
with DNA. This work illustrates a new regulatory mechanism for how proteins
- which from a mechanical engineer's perspective are nanomachines - operate."
Supported by the National Institutes of Health (NIH), the research involves two teams of scientists, one at Georgia Tech and Emory University in Atlanta, and the other at the Oklahoma Medical Research Foundation and University of Oklahoma Health Sciences Center in Oklahoma City. A paper describing the work was published in the May 8 issue of the journal Nature.
The researchers studied the activity of selectin molecules, a family
of proteins that helps control the adhesion of white blood cells - leukocytes
- used by the body to fight infection and repair injuries. Before they
can respond to injury or infection, leukocytes must first tether to and
then roll along the wall of a blood vessel. While tethered, the cells
receive signals instructing them to enter underlying tissue to fight pathogens
or repair injuries. The selectins control the first stage of that process,
causing the leukocytes to drop out of the bloodstream and begin attaching
to blood vessel walls.
In two separate but complementary experiments, the researchers found
evidence of catch bonds operating within the complex of P-selectin and
its ligand PSGL-1.
Using a custom-built atomic force microscope, researcher Bryan Marshall
applied piconewton-scale forces to a junction connecting P-selectin and
PSGL-1 molecules. Despite the difficulty of measuring such small forces,
he was able to demonstrate that increasing the force extends the lifetime
of the bonds under certain conditions.
"In one range, when we are increasing the force, we actually see
the lifetimes of the bonds increase," he said. "Once you get
past a certain point, the bonds behave like you would expect - when you
apply a larger force, things come apart faster."
In making the measurements, Marshall carefully picked up only the effects
of the interaction between P-selectin and PSGL-1 and shielded the instrument
from thermal fluctuations that produce forces greater than those he was
trying to measure. In several hundred measurements, Marshall applied forces
of less than 10 piconewtons - comparable to the force exerted by a beam
of photons leaving a laser pointer. He measured bond lifetimes as short
as a few thousandths of a second and as long as a few seconds.
The second experiment involved flow chamber tests designed to simulate
blood flow in the body. Oklahoma researchers perfused cells into the chamber
while controlling flow rates and shear forces. This allowed them to study
how adhesive bonds form and dissociate under the rolling interactions.
"We found that one range of forces, applied by increasing wall shear
stress, actually increased the lifetimes of adhesive bonds between the
cell adhesion molecule P-selectin and its ligand PSGL-1," said Dr.
Rodger McEver, Fred Jones Distinguished Scientist at the Oklahoma
Medical Research Foundation and adjunct professor of Biochemistry
and Molecular Biology at the University
of Oklahoma Health Sciences Center. "These observations confirmed
the atomic force microscopy results and reinforced their physiological
relevance in an experimental design that recapitulates cell interactions
in the circulation."
Understanding the phenomenon is important, McEver noted, because the
lifetimes of the adhesive bonds determine whether the white blood cells
form the rolling interactions with blood vessel walls that are necessary
for them to reach the point of inflammation.
The mechanism may play a vital role in allowing the leukocytes to do
their job without creating problems elsewhere in the body.
"Catch bonds may play a role in preventing the accumulation of white
blood cells in low-flow regions," said Marshall. "You really
want the adhesion to be very specific to where they are needed. If you
had really strong adhesion all the time, white blood cells would accumulate
in regions where they shouldn't. Catch bonds may be the body's way of
preventing white blood cells from lingering in stagnant backwaters in
the bloodstream where there is little flow."
While the existence of catch bonds has so far been confirmed in selectin
molecules, Zhu believes the phenomenon applies other instances in which
adhesive molecules interact in the presence of mechanical stress caused
by liquid flows. The adhesion of bacterial cells to the gastrointestinal
tract, for instance, may also rely on the mechanism to regulate when cells
should attach - and when they should not.
"One of our goals now is to demonstrate that catch bonds are universal
to at least several classes of molecules," added Zhu, who also holds
a faculty appointment in the Coulter
Department of Biomedical Engineering operated by Georgia Tech and
Emory University. "At the atomic level, we want to understand what
makes these interactions behave as catch bonds."
In addition to the researchers already mentioned, the team included Mian
Long and James Piper of Georgia Tech and Tadayuki Yago of the Oklahoma
Medical Research Foundation.
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